Plasma reactor with coil antenna of concentrically spiral conductors with ends in common regions

ABSTRACT

The invention is embodied in an antenna for radiating RF power supplied by an RF source into a vacuum chamber, the antenna including plural concentrically spiral conductors, each having a first end located in a first common region and a second end located in a second common region, and each being wound about a common axis passing through both regions, the regions being concentric with the axis, the conductors being substantially the same length, substantially the same shape, and substantially evenly spaced with respect to each other about the common axis.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 08/886,240 now U.S. PatNo. 6,297,468 filed Jun. 30, 1997. This application contains subjectmatter related to the following, commonly assigned U.S. applications:U.S. application Ser. No. 09/742,558 now U.S. Pat. No. 6,373,622 filedDec. 20, 2000 entitled PLASMA REACTOR WITH ANTENNA OF COIL CONDUCTORS OFCONCENTRIC HELICES OFFSET ALONG THE AXIS OF SYMMETRY, by Qian, et al.;U.S. application Ser. No. 09/742,051 now U.S. Pat. No. 6,369,348, filedDec. 20, 2000 entitled PLASMA REACTOR WITH COIL ANTENNA OF PLURALHELICAL CONDUCTORS WITH EQUALLY SPACED ENDS, by Qian,et al.; U.S.application Ser. No. 09/742,988, filed Dec. 20, 2000 entitled PLASMAREACTOR WITH COIL ANTENNA OF INTERLEAVED CONDUCTORS, by Qian, et al.,now U.S. Pat. No. 6,369,349.

BACKGROUND OF THE INVENTION

1. Technical Field

The invention is related to fabrication of microelectronic integratedcircuits with a inductively coupled RF plasma reactor and particularlyto such reactors having coiled RF antennas providing a highly uniformplasma distribution.

2. Background Art

Inductively coupled plasma reactors are employed where high densityinductively coupled plasmas are desired for processing semiconductorwafers. Such processing may be etching, chemical vapor deposition and soforth. Inductively coupled reactors typically employ a coiled antennawound around or near a portion of the reactor chamber and connected toan RF power source. In order to provide a uniform etch rate ordeposition rate across the entire surface of a wafer, the plasma densityprovided by the coiled antenna must be *uniform across the surface ofthe semiconductor wafer. One attempt to provide such a uniform field isto wind the coiled antenna in a flat disk parallel to overlying-thewafer, as disclosed in U.S. Pat. No. 4,948,458 to Ogle. This concept isdepicted in FIG. 1.

One problem with the flat coiled antenna of FIG. 1 is that there is alarge potential difference between the center of the antenna and thecircumferential edge thereof, with the result that the plasma can have ahigh ion density or “hot spot” over the center of the wafer and a lowerion density at the wafer periphery., This in turn causes the etchrate—or deposition rate—to be nonuniform across the wafer surface.

One way of ameliorating this problem is to limit the power applied tothe antenna coil to a few hundred watts so as to minimize the plasmanon-uniformity. This approach is not completely satisfactory because itlimits the etch rate (or deposition rate), thereby reducing throughputor productivity of the reactor, and moreover does not solve the problemof process non-uniformity across the wafer surface.

Another problem with inductively coupled reactors is that any highvoltage applied to the antenna coil leads to capacitive coupling of RFpower to the plasma. In other words, capacitive coupling of RF powerfrom the coiled antenna to the plasma increases with the voltage on thecoiled antenna. Such capacitive coupling can increase the ion kineticenergy which makes it difficult for the user to precisely control ionkinetic energy and thereby control sputtering rate or etch rate.Capacitive coupling is particularly pronounced in the flat disk coilantenna of FIG. 1.

Therefore, there is a need for an inductively coupled plasma reactorhaving a coiled antenna which provides a highly uniform plasma acrossthe wafer surface at high power with only minimal capacitive coupling.

SUMMARY OF THE INVENTION

The invention is embodied in a coil antenna for radiating RF powersupplied by an RF source into a vacuum chamber, the antenna includingplural concentrically serial conductors, each having a first end locatedin a first common region and a second end located in a second commonregion, and each being wound about a common axis passing through bothregions, the regions being concentric with the axis, the conductorsbeing substantially the same length, substantially the same shape, andsubstantially evenly spaced with respect to each other about the commonaxis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagram of a coiled antenna for an inductivelycoupled plasma reactor of the prior art.

FIG. 2 is a simplified diagram of a coil antenna having its windingsconnected in parallel across an RF source.

FIG. 3A is a top view of a flat disk coil antenna for a plasma reactorin accordance with a first embodiment of the invention.

FIG. 3B is a top view of a flat disk coil antenna corresponding to FIG.3A but having a greater number of windings.

FIG. 4 is a side view corresponding to FIG. 3A.

FIG. 5 is a perspective cut-away view of an inductively coupled plasmareactor employing the coiled antenna of the embodiment of FIG. 3A.

FIG. 6 is a perspective view of a cylindrical coil antenna in accordancewith a second embodiment of the invention.

FIG. 7 is a perspective view of a coil antenna in accordance with athird embodiment of the invention which is a variant of the cylindricalcoil antenna of FIG. 6 in that the cylindrical coil is continued overthe ceiling of the reactor.

FIG. 8 is a perspective view of a coil antenna in accordance with afourth embodiment of the invention having a dome shape.

FIG. 9 is a perspective view of a coil antenna in accordance with afifth embodiment which is a variant of the cylindrical coil antenna ofFIG. 6 and the dome antenna of FIG. 8.

FIG. 10 is a perspective view of a coil antenna in accordance with asixth embodiment having a truncated dome shape overlying a truncateddome-shaped ceiling of a plasma reactor.

FIG. 11 is a perspective view of a coil antenna in accordance with aseventh embodiment having a truncated dome-shaped portion overlying atruncated dome-shaped reactor ceiling and a cylindrical portionsurrounding the reactor side wall.

FIG. 12 is a perspective view of an embodiment of the invention having atruncated dome ceiling forming a chamfered corner along thecircumference of the ceiling.

FIG. 13 is a perspective view of a variation of the embodiment of FIG.12 including a cylindrical winding.

FIG. 14 is a perspective view of an embodiment of the invention having ashallow or partial dome-shaped ceiling.

FIG. 15 is a perspective view of a variation of the embodiment of FIG.14 including a cylindrical winding.

FIG. 16 is a perspective view of an embodiment of the invention having ashallow dome-shaped ceiling with a chamfered corner along itscircumference.

FIG. 17 is a perspective view of a variation of the embodiment of FIG.16 including a cylindrical winding.

FIG. 18 contains superimposed graphs of ion current measured at thewafer surface as a function of radial position from the wafer center forvarious types of reactors of the prior art and corresponding graphs fora reactor incorporating the present invention.

FIG. 19 contains superimposed graphs of ion current measured at thewafer surface as a function of reactor chamber pressure for differentreactors of the prior art.

FIG. 20 contains superimposed graphs of ion current measured at thewafer surface as a function of reactor chamber pressure for -differentreactors of the prior art and corresponding graphs for a reactorincorporating the present invention.

FIG. 21 contains superimposed graphs of ion current measured at thewafer surface as a function of reactor chamber pressure for differentreactors of the prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In an inductively coupled plasma reactor having an RF antenna coiladjacent the reactor chamber, it is a goal of the invention to reducethe voltage on the coil. One possible approach to reduce coil voltage isto reduce the amount of inductance in the winding of the coil antenna.This would reduce the potential V across each winding (since V=L di/dt,where L is the winding inductance and i is the winding current), thisreduction in electric potential reducing capacitive coupling to theplasma. FIG. 2 illustrates one way of accomplishing this by connectingall of the coil windings 10, 12, 14 in parallel across the RF powersource 16, 18 via conductors 20, 22. One end 10 a, 12 a, 14 a of eachwinding is connected to the conductor 20 while the other end 10 b, 12 b,14 b is connected to the other conductor 22. The problem is that the gap24 between the conductors 20, 22 gives rise to a discontinuity in the RFfield. Thus, for example, in a plasma etch reactor employing the coiledantenna of FIG. 2, the discontinuity of the coil can often causeazimuthal asymmetry in the plasma density across the wafer surface.Accordingly, the coil antenna of FIG. 2 does not provide a uniformplasma density and therefore does not fulfill the need.

Referring to FIGS. 3A and 4, a coil antenna 30 overlies the ceiling of areactor chamber 31, the coil antenna 30 having plural concentric spiralwindings 32, 34, 36 connected in parallel across a capacitor 62 and anRF source 64. The windings 32, 34, 36 have inner ends 32 a, 34 a, 36 anear the center of the spirals and outer ends 32 b, 34 b, 36 b at theperipheries of the spirals. The inner ends 32 a, 34 a, 36 a areconnected together at a common apex terminal 38. In the preferredembodiment, the common apex terminal 38 is connected to ground while theouter winding ends 32 b, 34 b, 36 b are connected to the RF source 64.As shown in FIG. 4, the straight central inner arms of the windings 32,34, 36 preferably wick vertically upwardly away from the reactor top tothe apex terminal 38 by a vertical distance v of about 2 cm. FIG. 3Billustrates a 5-winding version of the coil antenna of FIG. 3A,including concentric windings 32, 33, 34, 35, 36 with inner endings 32a, 33 a, 34, 35 a, 36 a and outer endings 32 b, 33 b, 34 b, 35 b, 36 b.

FIG. 5 illustrates an inductively coupled plasma reactor including acylindrical vacuum chamber 50 having a flat disk insulating ceiling 52,a grounded conductive cylindrical side wall 54, a gas supply inlet 56and a wafer pedestal 58. A vacuum pump 60 pumps gas out of the vacuumchamber. The coil antenna 30 of FIG. 3A rests on the ceiling 52. An RFpower source 64 applies power through the capacitor 62 to the outerwinding ends 32, 34, 36 while the common terminal 38 is grounded. A biasRF power source 66, 68 is connected to the wafer pedestal 58 to controlion kinetic energy.

In a preferred implementation of the embodiment of FIG. 5, the circularwindings become straight radial arms terminating in the apex terminal38, the arms extending along a radius r (FIG. 5) of about 2.5 cm. Theoutermost one of the windings 32, 34, 36 has a radius R (FIG. 5) ofabout 35 cm in those cases in which the wafer diameter d (FIG. 5) isabout 20 cm. The height h (FIG. 5) of the coil antenna above the waferis preferably about 5.0 cm to 7.5 cm. Preferably, each one of the coilwindings 32, 34, 36 makes 1.5 turns. The number of windings per lengthof radius, which in the embodiment of FIG. 5 is 1.5/26 cm⁻¹, may bechanged so as to desirably adjust the plasma density distribution acrossthe wafer surface.

FIG. 6 illustrates a cylindrical version 60 of the coil antenna 30 ofFIG. 3A, which also has plural concentric spiral windings 32′, 34′, 36′each wrapped around an insulating portion of the cylindrical side wall54 of the reactor of FIG. 5. The plural concentric windings 32′, 34′,36′ have respective inner ends 32 a′, 34′, 36 a′ terminating in a commonapex terminal 38 a, as well as outer ends 32 b′, 34 b′, 36 b′terminating equidistantly from each other at locations about the lowersidewall of the reactor chamber. FIG. 7 illustrates another version ofthe cylindrical antenna 60, in which inner ends 32 a′, 34 a′, 36 a′ ofthe antenna 60 continue in spiral fashion across the top of the reactorin a form much as in FIG. 5 to the common apex terminal 38 b, thusforming a continuous single cylindrical coil antenna 70 extending notonly over portions of the cylindrical wall 54 but also over the ceiling52 of the reactor. Preferably, each winding 32′, 34′, 36′ makes a smoothtransition at the corner between the ceiling and the cylindricalsidewall, in the manner illustrated in the drawing.

FIG. 8 illustrates a dome-shaped version 80 of the coil antenna 30 ofFIG. 3A for use with a version of the reactor of FIG. 5 in which theceiling 52 is dome-shaped. FIG. 9 illustrates how the dome-shaped coilantenna 80 may be integrated with the cylindrical shaped coil antenna 60to form a single antenna 90 covering both the dome-shaped ceiling andcylindrical side wall of the reactor of the embodiment of FIG. 8. Thewindings make a smooth transition from the dome-shaped ceiling to thecylindrical sidewall in the manner illustrated in the drawing. FIG. 10illustrates a modification of the coil 80 of FIG. 8 in which thedome-shaped ceiling is truncated so as to have a flattened apex. FIG. 11illustrates a modification of the coil of FIG. 9 in which thedome-shaped ceiling is truncated so as to have a flattened apex.

The windings 32, 34, 36 are spaced from one another by a sufficientspacing to prevent arcing therebetween. In order to provide an azimuthalsymmetrical RF power feeding and minimum potential difference betweenadjacent winding along the entire lengths thereof, all windings 32, 34,36 preferably are of the same length. In the illustrated embodiments,the spacings between windings are equal and are uniform throughout theantenna coil. However, the invention may be modified by varying thewinding-to-winding spacings so as to be different at different locationsor to differ as between different pairs of windings.

While the invention has been described with reference to preferredembodiments having three concentric spiral windings 32, 34, 36, otherembodiments of the invention may be made with as few as two suchwindings, four such windings or any desired number of windings, providedthe requisite winding-to-winding spacing is maintained to avoid arcing.A greater number of spiral windings provides a more uniform RF field andin some cases more uniform plasma ion density across the wafer surface.

FIG. 12 illustrates a variation of the embodiment of FIG. 10 in whichthe ceiling 52 has a central flat region 52 a surrounded by an annularchamfer 52 b which provides a smooth transition from the horizontal flatregion 52 a to the vertical side wall 54. This in turn helps thewindings 32, 33, 34, 35, 36 make a smooth transition as well. An annularportion of the coil antenna overlies and conforms with the cornerchamfer. Furthermore, the embodiment of FIG. 12 has five concentricwindings 32, 33, 34, 35, 36 with outer ends 32 b, 33 b, 34 b, 35 b, 36.FIG. 13 illustrates a variation of the embodiment of FIG. 12 in whichconcentric windings 32′, 33′, 34′, 35′, 36′ make a smooth transition atthe corner chamfer from the flat portion of the ceiling 52 to thecylindrical side wall, each of these windings including a first portionoverlying the flattened central part 52 a of the ceiling 52, a secondportion overlying the corner chamfer 52 b of the ceiling 52 and a thirdportion wrapped around the cylindrical side wall 54. The winding outerends 32 b′, 33 b′, 34 b′, 35 b′, 36 b′ defining the bottom of the coilantenna are disposed at about the same height as the top of the waferpedestal 58 and are connected to the output terminal of the RF sourcethrough the capacitor 62. The pitch of the windings may vary withlocation so that, as one example, the windings on the top may be at onepitch while the winding along the cylindrical side wall may be at adifferent pitch, thus providing greater control over the plasmaformation.

FIG. 14 illustrates a variation of the embodiment of FIG. 12 havingflattened dome-shaped ceiling, whose arc subtends an angle substantiallyless than 180 degrees, for example about 90 degrees. In contrast, forexample, the dome-shaped ceiling of FIG. 10 subtends approximately 180degrees of arc. FIG. 15 illustrates a variation of the embodiment ofFIG. 13 also having flattened dome-shaped ceiling, whose arc subtends anangle substantially less than 180 degrees, for example about 90 degrees.

FIG. 16 illustrates an embodiment combining a flattened central dome 52a′ like that of FIG. 14 with an outer corner chamfer 52 b′ like that ofFIG. 12. FIG. 17 illustrates a variation of the embodiment of FIG. 16 inwhich the windings make a smooth transition at the corner chamfer 52 bfrom the ceiling 52 to the cylindrical side wall 54. The embodiments ofFIGS. 12-17 are illustrated as having 5 concentric windings each, incontrast with the 3 concentric windings of the embodiments of FIGS.3-11. The invention can be implemented with any suitable number ofconcentric windings.

Advantages of the Invention

The parallel arrangement of the windings 32, 34, 36 of the coil antenna30 of FIG. 3A reduces the potential across each winding, as compared to,for example, using only one winding, and therefore reduces thecapacitive coupling (as explained above with reference to the example ofFIG. 2). In addition, the coil antenna of FIG. 3A provides uniformplasma density over the wafer, as compared previous techniques forexample, as there are no discontinuities of the type discussed abovewith reference to the example of FIG. 2 (e.g., in the RF field). Suchimproved uniformity is not limited to etch applications, but is alsorealized when the invention is used in other plasma-assisted processes,such as chemical and physical vapor-deposition of coatings. Further, ascompared to prior art FIG. 1, not only is the potential across eachwinding reduced, but also the current flowing in the parallel windingsof the invention is spatially distributed over the reaction volume in amuch more uniform fashion.

Preferably, each of the windings 32, 34, 36 have the same length andtheir outer ends 32 b, 34 b, 36 b terminate at points equidistant fromeach other about a circularly symmetric reaction chamber, furtherenhancing uniformity. Preferably, the winding inner ends 32 a, 34 a, 36a terminate at the geometric center of the coil antenna because the apexis located at the geometric center of the coil, which preferably hasgeometric circular symmetry. Preferably also, this geometric antennacenter is made to coincide with the axis of symmetry of a circularlysymmetric reactor chamber. Also, the winding inner ends 32, 34 a, 36 aare preferably spaced equidistantly away from each other for a limitedradial distance as they approach the apex terminal 38 a. Further, thewindings are spaced from each other as uniformly as possible at least inflat configurations of the invention such as the embodiment of FIG. 3A;while in non-flat configurations such as the embodiment of FIG. 8,smoother variations and spacings with radius from the geometrical centermay be made to compensate for chamber geometry.

As a result, the RF power applied to the coil antenna of FIG. 3A neednot be limited as in the case of the coil antenna of FIG. 1. Indeed, thecoil antenna of FIG. 3A can operate with 3000 Watts of RF power at 13.56MHz, while the coil antenna of FIG. 1 must be limited to about 300 wattsto prevent failures due to the non-uniform field coverage. The increasein RF power afforded by the coil antenna of FIG. 3A provides higher etchrates in a plasma etch reactor, higher deposition rates in a chemicalvapor deposition reactor. Thus, the invention not only provides greaterprocessing uniformity across the wafer surface but also provides greaterthroughput or productivity.

The invention provides a greater uniformity of ion density across thewafer surface, a significant advantage. This is illustrated in thesuperimposed graphs of FIG. 18. The curves in FIG. 18 labelled A1, A2,A3 and A4 represent measurements of ion current at the wafer surface inmilliAmperes per square centimeter as a function of distance from thewafer center in centimeters for a reactor employing the coil antenna ofthe invention depicted in FIG. 3A with a reactor chamber supplied withchlorine gas at an applied RF power level of 2000 Watts on the antennacoil, no RF bias power applied and the chamber maintained at a pressureof 2 milliTorr, 6.2 milliTorr, 10 milliTorr and 4 milliTorr,respectively. The smallest deviation in ion density, namely 2% in thecurve labelled A1, is obtained at 2 milliTorr. The uniformity percentagerepresents the change in current density (vertical axis) across thewafer divided by two times the average current density in that range. Incontrast, a reactor sold by manufacturer #1, whose performance isdepicted by the curve labelled B in FIG. 18, had a deviation in plasmaion density of 4.5% across the wafer surface at the same applied RFpower level and no RF bias power applied and a mixture of 50 parts ofchlorine and 20 parts helium. A reactor sold by manufacturer #2, whoseperformance is depicted by the curve labelled C in FIG. 18, had adeviation in plasma ion. density of 9% under similar conditions. Areactor sold by manufacturer #3, whose performance is depicted by thecurve labelled D in FIG. 18 had a deviation of 11% in plasma ion adensity across the wafer surface. A reactor sold by manufacturer #4,whose performance is depicted by the curve labelled E in FIG. 18, had adeviation in plasma ion density across the wafer surface of 26% at anapplied power level of 900 Watts on the antenna coil. The foregoing datais summarized in the following table:

TABLE I Ion Applied Current Power Pressure Ion Density Plasma Reactor(mA/cm²) (Watts) (mTorr) Gas Deviation Invention 12.8 2000 2 Cl  2%Manufacturer #1 17 2000 1.2 50 Cl/ 4.5%  20 He Manufacturer #2 11.4 20002 Cl  9% Manufacturer #3 7.6 1450 2 Cl 11% Manufacturer #4 11.5  900 5Cl 26%

The invention provides a greater stability of ion density over a largerange of chamber pressures, a significant advantage. The performance oftwo plasma reactors of the prior art sold by manufacturers #2 and #3 aredepicted by the superimposed curves labelled C and D, respectively, inFIG. 19. The vertical axis is a normalized measured ion current at thewafer surface while the horizontal axis is the chamber pressure inmilliTorr. The manufacturer #2 plasma reactor (curve C) has a deviationof 23% in ion current over a pressure range from 2 to 5 milliTorr. Themanufacturer #3 reactor (curve D) has a deviation of 40% in ion currentover the same pressure range. The performance of the invention inaccordance with FIG. 3A and of other prior art reactors is depicted inthe superimposed graphs of FIG. 20. The curves labelled A1, A2, A3 andA4 depict the ion current measured at the wafer surface in the reactorof the invention at distances of 0 cm, 2.9 cm, 5.9 cm and 8.8 cm,respectively, from the wafer center. These curves show that thedeviation in ion density using a reactor of the invention is no morethan 10% across the same pressure range. The reactor sold bymanufacturer #1, whose performance is depicted by the curve labelled Bin FIG. 20, had a deviation of 22% across a much narrower pressurerange. A reactor sold by manufacturer #5, whose performance is depictedby the curve labelled F in FIG. 20, had a deviation in ion density of 45across a similar pressure range (2-5 milliTorr). The reactor sold bymanufacturer #4, whose performance is depicted by the curve labelled Ein FIG. 21, had a deviation in ion density of 25% across the narrowerpressure range of 0.5 to 2.0 milliTorr. The foregoing experimentalmeasurements relating to stability of ion density over change in chamberpressure are summarized in the following table:

TABLE II Ion Ion Applied Density Current Power Pressure De- PlasmaReactor (mA/cm²) (Watts) (mTorr) Gas viation Invention 10 2000  2-10 Cl10% Manufacturer #1 17 2000 7-2 50 Cl/ 22% 20 He Manufacturer #2 11.42500 2-5 Cl 23% Manufacturer #3 7.6 1000 2-5 Cl 40% Manufacturer #4 11.5 300  2-10 Cl 25% W (source)  30 W (bias) Manufacturer #5 15 1000 2-5 N45%

The foregoing experimental data show that the invention provides astability in ion density over changes in pressure over twice that of thebest reactors of the prior art and at least four times that of otherreactors of the prior art.

While the invention has been described in detail by specific referenceto preferred embodiments, it is understood that variations andmodifications may be made without departing from the true spirit andscope of the invention.

What is claimed is:
 1. An antenna for radiating RF power into a vacuumchamber, the antenna comprising: plural concentrically spiralconductors, each having a first end located in a first common region anda second end located in a second common region, and each being woundabout a common axis passing through both regions, said regions beingconcentric with said axis, said conductors being substantially the samelength, substantially the same shape, and substantially evenly spacedwith respect to each other about said common axis.
 2. The apparatus ofclaim 1 in which said the second common region is radially outwardlydisplaced from said axis further than said first common region, wherebysaid conductors spiral radially outwardly away from said axis.
 3. Theapparatus of claim 1 in which the first and second common regions areaxially displaced with respect to each other along the direction of saidaxis, whereby said conductors spiral axially along the direction of saidaxis.
 4. The apparatus of claim 3 wherein said plural conductors eachform a solenoidal helix of plural conductors, each solenoidal helixcomprising plural winding displaced from adjacent windings by awinding-to-winding displacement, each said solenoidal helix beingdisplaced from each adjacent solenoidal helix by a distance less thansaid winding-to-winding displacement.
 5. The apparatus of claim 3 inwhich one of said regions is outside the other with respect to saidaxis.
 6. The apparatus of claim 2 in which at least one of said commonregions is orthogonal to said common axis.
 7. The apparatus of claim 1in which said first and second common regions lie in respective planesparallel to each other.
 8. The apparatus of claim 1 in which said firstand second common regions are coplanar.
 9. The apparatus of claim 1 inwhich the distance between adjacent conductors increases with distancefrom the common axis.
 10. The apparatus of claim 1 in which the distancebetween adjacent conductors remains substantially the same throughouttheir lengths.
 11. The apparatus of claim 1 wherein said first ends aregenerally located at a first common radius from said axis and areangularly displaced from one another.
 12. The apparatus of claim 11wherein an angular distribution of said first ends is uniform.
 13. Theapparatus of claim 11 wherein said second ends are generally located ata second common radius from said axis and are angularly displaced fromone another.
 14. The apparatus of claim 13 wherein an angulardistribution of said second ends is uniform.
 15. The apparatus of claim13 wherein said first and second regions correspond to respective radiallocations about said axis, wherein said second region is located at aradius greater than that of said first region.
 16. The apparatus ofclaim 15 wherein said plural conductors conform at least partially to atleast one of the following shapes: dome shape; a flat disk shape. 17.The apparatus of claim 13 wherein said first and second regions liegenerally within respective plane generally orthogonal to said axis andaxially displaced from one another along the direction of said axis. 18.The apparatus of claim 17 wherein said plural conductors conform atleast partially to at least one of the following shapes: dome shape;solendoil cylindrical shape.
 19. The apparatus of claim 17 wherein saidfirst and second ends are located within said respective axially offsetplanes and at the same radius from said axis, whereby each of saidplural conductors conforms to a right cylindrical solenoid.
 20. Theapparatus of claim 19 wherein said first ends are generally angularlyaligned with said second ends.